An anonymous reader quotes a report from Recode: Unlimited data plans are slowing down mobile speeds for Verizon and AT&T customers, according to data released today by mobile network measurement company OpenSignal. Verizon and AT&T reinstated their unlimited plans in February to compete with T-Mobile and Sprint, which have long offered unlimited data plans, and have since seen a deluge of demand. Greater data demand -- either more data usage or more customers -- means slower speeds. Think of it as increased traffic on a highway. Verizon and AT&T also have nearly double the subscribers of T-Mobile and Sprint, so changes in their offerings hit their networks harder. Both Verizon and AT&T saw a notable decline in speeds after introducing unlimited plans. T-Mobile and Sprint have been able to gradually account for the increase in data demand, so their speeds weren't negatively affected this year -- indeed, they both got faster since OpenSignal's February report. Verizon and T-Mobile were basically tied for speeds at the beginning of this year. Now, T-Mobile has taken the lead with an average LTE download speed of 17.5 Mbps, compared with Verizon's 14.9 Mbps. Here's a good comparison of the unlimited plans currently offered by the "Big 4" carriers.